Induction accelerator



March 20, 1951 D. w. KERST INDUcToN AccELERAToR 2 Sheets-Sheet 1 Filed March 22, 1946 March 20, 1951 D. w. KERST 2,545,958

INDUCTION ACCELERATOR Filed March 22, 1946 2 Sheets-Sheet 2 Patented Mar. 20, 1 951 is disposed between two generally circular pole pieces having main field producing coils associated therewith. The field strength relationships, to provide an equilibrium orbit within the tube, must be such that the rate of change of average field strength within the orbit is double that at the orbit. This may be accomplished by using pole pieces with outwardly tapered or diverging configurations as shown and fully described in my above-mentioned Patent 2,297,305, with nothingV` more than the main energizing coils associated with the pole pieces; or it may be accomplished by use of main and auxiliary (or flux forcing) coils to provide the desired relationships, as is fully disclosed in my copending application Serial No. 627,481, filed November 8, 1945.

The pole pieces are preferably part of a magnetic structure including a rectangular frame, and the magnetic or main energizing coils are preferably energized by an alternating current of considerable amplitude. These coils may be connected in parallel with condensers, for example, in an arrangement resonant at or about 180 cycles per second, and energized from an alternating current generator of the same frequency, the resonating circuit providing for high circulating currents. The electron injecting mechanism is operated, as by excitation of a thyratron grid by voltage from a pick-up or peaking strip in the field -of the main magnet coils to inject the electrons into the tube at the beginning of the rising quarter cycle of current on the positive side. The injector structure is arranged to inject the electrons into the tube in a direction substantially tangent to the circular path, and with substantial initial energy of at least several electron volts and preferably of the order of 1,000 electron volts or more. The general principles of structure and design of injectors are shown in the abovementioned issued patents and publications, and in my copending application Serial No. 617,255, which issued February 21, 1950 as Patent No. 2,497,891, so that the injector will be passed for the time being and returned to later in connection with details of particular relation to this invention.

Interaction between the electrons, moving with their initial energy, and the vertical lines of force of the varying eld set up through the tube cause them to be accelerated both radially and circumferentially. Where the injector is outside of the equilibrium orbit, as here, the eld strength relationships are arranged such that the electrons are given an inward radial acceleration and spiral in toward the equilibrium orbit as the increasing magnetic field accelerates them circumferentially. This initial radial acceleration may be increased, if desired, by use of an arrangement of the kind shown in the co-pending Adams application Serial No. 607,416, which issued June 14, 1949 as Patent No. 2,473,123. A,

lWhen the electrons reach the equilibrium orbit they ,continue to circulateat or near this orbit with an acceleration which continues during the entire rising portion of the positive quarter cycle of lenergizing current, as the magnetic eld is increasing during this time; and since the electrons may have described several thousand revolutions in the time during which they are thus subjected to `a continually increasing magnetic field strength, their energy may be Worked up to millions of electron volts. During the time the electrons are describing revolutions in and about the equilibrium orbit, maintenance of the electrons at or near such orbit is effected, in the case of radial stability, by proprtioning of the iield relationships such that the average change of field strength throughout the area within the orbit is double the change of field strength at the orbit itself; and vertical (or axial) stability results from the curvature of the lines of force in a vertical plane passing through the orbit.

After the electrons have traveled around in or near the equilibrium orbit for asA many revolutions as may be necessary to give them the desired high kinetic energy, they may then be causd to follow a gnerally spiraling instantaneous path, as for example an outwardly spiraling path, to some point or space where there is an action effecting the desired ultimate use of the electron energy. Most betatron installations heretofore made have provided a target of suitable material mounted at an appropriate point in the tube (on the injector structure II, for example) and caused the accelerated electrons to impinge against this target to effect generation of X-rays, as is more fully described in some of the abovementioned patents and publications, or in the co-pending application Serial No. 534,060, which issued August 17, 1948 as Patent No. 2,447,255, of myself and Serber. The present invention, however, is concerned with means spiraling the accelerated electrons to a space in the tube where the field strength is very vlow relative to other areas in the tube, to cause a relatively sudden change in direction of electron movement enabling the electrons to be proected from the tube in an electron beam.

I secure a space of very low relative eld strength by placing in the tube at one point a eld diminishing element or member I2, referring to the embodiment of my invention illustrated in Figures 1-3. This member must be of high magnetic permeability, as ferrous material, the preferable embodiment illustrated in Figures 1-3 being made of laminations of transformer core iron with the laminations vertical (speaking with respect to an induction accelerator arranged with the axis of rotation vertical, as is common). The member I2 is generally U-shaped or horse-shoe shaped in cross section, with inwardly extending arm portions I2a and I2b providing a slot therebetween in the plane of the equilibrium orbit, this member having a connecting or back portion I2c having a thickness (in the plane of the equilibrium orbit) of the same order as the depth of the slot, preferably being at least equal to such depth. This portion I2c must also be of substantial dimensions in a direction transverse to the plane of the slot, since its purpose is to carry, without exceeding saturation and preferably at levels below saturation, all of the lines of force which would normally exist in the entire area comprising a projection of this member I2 in the plane of the equilibrium orbit. That is, lines of magnetic force entering the top of the deflecting member I2 should substantially entirely take a path through the back or solid portion of this member rather than across the gap provided by the slot; and the lines of force will take such path if it is made of sufficiently low reluctance by the provision of sufficient iron in the unslotted portion of the member. The result is that the space in the slot between the portions I2a and I2b, and even slightly outside of this slot as a result of the attraction to such portions of lines of magnetic force, comprises a space of very greatly reduced field strength at any time relative to the field strength existing in adjacent areas in the tube. In order to secure satisfactory results,

the eld strength in the slot should notfexceed yhalf of the leldfstrengthlinadjacentareas, and preferablyshouldbe dfl-th'e order-'of Aonly 5%'01' '10%,fso that forl-allipractical purposes-it is a Aspace of su-bstantially no `magnetic lleldstrength.

The Walls bf lthe Aslot are substantiallyfstraight, by 'which is meant-said walls are without sharp deviations, although the walls need not necessarily belperfectly lstraight as they areillustrated in Fig. 2.

As will be :readilyffap'parerit from -'a 'consideravzwtion vof'".Eigure 1,-'the equilibriuni'lorbit-lies' well to `the inside (radially speaking) 'of z"lille-T130'si-tion of the deectn'gfele'rrient T2, Wliihiis placed in the-tube j ustfaheadi-(speakihg IWith-1es1`nect'to the direction df travel -of 'thei'elec'tronm-ff *a shouldered p'ortion provided with a window -lia rfa- -cilitating f-eXit :of the electrons from the Atube. This window `-ma'y consist, forfexample, `of Y: ai piece of-thin y aluminum or `othermetal-foil v Ia, over va Lsuitable -opening 711011 in fthe glass or -porcel'ain wall AVif the tube. AIn operation of this Adevice 'thefelectrons are-injected at asuitable time A"by 'the structure H, spiraledfinward to the'equilibrh um orbit, and accelerated in yaf-large numberffof turns, -as vhas been heretoforedescribed A'Ihey lare then causedto lspiral-awayffrom'the equilibri- Aum orbit,A outwardly in this case, by lany-'suitable arrangementv 'Ior`changin'g the'ieldstrengthTelationships, as is more fully described'fimmylarticlein `'thef Reviewl ofScientic Instruments, vol. 13, pages '387-394 -(September, 1942) or in nmy 'PatentvNa 2,394,070, "which issued February -5, 194:6. The outwardfpathlof electronsis-irr aspiral 'with continually vdivergn',; turns, with f at y:least several millimeters, and sometimes aslmuch as eightor ten millimeter difference, "between the 'radiiof the la-st two "turns inthe-plane of the deecting vmember 'Ai/'2. Accordingly,"even`lr though the l'deflectin'g member'tends to collect lines 0f force from several times its own area, the electrons are substantially unaffected in their path if they are at least 'several millimeters Yfrom `the open end of the slot in the deflector member on the turn immediately 'preceding that in which they pass into the space provided by the slot. In this last turn the electrons suddenly find'themselves y'in `Yan area 'of what `is fsubstantially. zero v"field streh'g'thfso that"tl'er`e' is'littl'e or noinward radial force to counteract centrifugal force, and the electrons straighten out their path and move toward and through the foil Window Illa to provide a well collected beam of electrons which may be used for therapeutic, irradiating, or other purposes.

.While a deflecting means can be made of any suitable construction effecting the desired very great reduction in field strength in a certain space, I prefer the particular construction illustrated in Figures 1-3. Since the back or unslotted end is normally in a region of a magnetic field strength of at least 2,000 to 3,000 gauss and since it will carry leld strength from at least double or several times its own cross section, it must be thick and of material of high magnetic permeability. The particular deiiector means illustrated has a total radial thickness of two centimeters, with a slot one centimeter deep and two millimeters wide (transverse to the plane of the equilibrium orbit), the over-all dimensions transverse to the slot being one centimeter and the length of the deflector member (along the line of movement of the electrons) being about l to l5 centimeters.

In order to use a beam extracting arrangement ofrth'e kind :just fdeseribedit is necessary that the electrons be initially injectedlfromian injecti 'stiuctllre v Which *Will t interfere "vi/ith* the ultimate -'s'piraliiig 5 path f ythe 'el'e'ctroiis-atfthe end .of their acceleration period; "-i. e.,-=`eith`erfin side :the @equilibrium orbit or fab'ove orbel'ow -the Aplane offtheorbit, or both, 'if outsidetheequilibriiim orbit, where th'elultimate spi-ralihg-islto be outwardly. 'While thespaceofreduced-'magnetic eld strength"couldA be 4prvid'ed to theinside of the equilibrium obit Fand Athe 'fel'ectii'is iSpi-aled fin'atthe end ofl their 'acceleratim `"I'-1i"ave"Sh'OWn an arrangement Where this Spaceisbuts'ideef the -orbit'an'd the velec'tions arels'piraled 'outwardlw'as kthisresiilts laigenpitch changeslbetwehtiirris `O'f the instantanecuselcctrn pathfdiin'gspir-'aling, and inv 'easierh'andlingff lthe beam on the Way out of 'the ltube. Where the electrons'are Aspiraled outwardly it maybe-preferable to l"use an in'j ector structure accomplishing injection t'o the-inside 'of theequilibrillrnl orbit in'- place 4r 0f the injector' structure fshown. In order vto Jinliectthe electrons lnearer the equilibrium orbit from r'1an injector structure outside of such orbit without at 'the same time-interfering withthe outward spiraling at the end of theacceleratiohperiod I have providedlwhat'is -in effect a-dOuble-i structure, as'- may be best ls'een in -Figuresl I and'G. Vflhls structure comprises Y upper? andllowe'r Adouble portions l-cr-*sectlris `here Aidentified" in generales l i )and ii Ea'chfse'ction comprises a lai'nent, as ythe.'filament ii a; fa -cup member l ibshielding plates Hc 'and 4M; land 4'app'ropr-ia-tearticulating and connectingl -means. v"As may be A best v`seen f'i'n Figure "f G, the n"shi'eldir-ig lmeans cups Yand17 filaments are spaced -apart so-a`s to `-`-pr`c` vite a centralfslct 'at 'half its 'center vcin'ciiing with the lplane-Of the 'equilibrium orbit. v?Th"e l'ilamen-ts 'fare E'cen- 'statly `heated, and at the appropriate time 'or injection, a high negative pote'n'tiaL -as several thousand volts rnegative v"with respect 'to the Vlilament, is applied to tthe cups, resultingiih'elec- -trons streaming #out-ffrnithe filaments with-"substantial initial-:energy and i generally tangent `vto the ultimate equilibrium orbit although lslightly tion. `rl-h'e jinward'-accele'ratin'g'forcelissuch' that vthe electronsclear vthe injectork structurelas they come laround "in `:the ii'rst ftu'rn, YTand they th'en 'Spiral iinwardly 't'o ithe Lequilibrium orbit where 'theirLvruearanulrauiaiosciuaupns-are gradually dampened until they are revolvingfairly'fstable in the equilibrium orbit during the major p0rtion of their acceleration. When sufficient acceleration has been effected and they are started in their instantaneous outward spiraling path, they remain in or so close to the plane of the equilibrium orbit that their path for the last couple of turns or so passes through the slot in the injector structure il, so that this structure, although to the outside of the equilibrium orbit, provides no impediment to the ultimate path desired.

Another embodiment of my delecting arrangement is illustrated in Figure 4, comprising a substantially rectilinear block I2 of high magnetic permeability. This block may, for example, be about one centimeter high, one-half centimeter thick (radially of the device) and several centimeters, as three to fteen centimeters, long. This attracts to itself suicient lines of force to provide a space, within a couple of millimeters or so of its inner surface, wherein the field strength is less than half that of adjacent open areas, sufficient to straighten out the spiral path of the Je'ctor 7 electrons and cause them to pass out of the Window I0a in the tube l0.

Still another deecting member embodying my invention is illustrated in Figures 7 and 8, this embodiment of my inventions being more analogous to the member illustrated in Figures 1-3 and described in connection therewith. While the embodiment of Figures 7 and 8 is illustrated as a single piece of material I2, again generally U-shaped, it will be understood that it may be of laminations arranged parallel to the axis of circulation of the electrons in a plane passing through such axis. This form of deflecting arrangement diiers from that of Figures 1-3 primarily in that the back or base portion I2C is considerably wider (in a direction parallel to the axis of the induction accelerator) than the outer dimensions across the leg portions I2a, and I2b", providing suitable iron for the lines of force and their changes of direction without reaching or substantially exceeding saturation. Any exceeding of saturation in the iron of the deecting member, or any portion thereof, tends to result in lines of magnetic force passing across the slot therein, so that this must be minimized in so far as is possible.

The deflecting arrangement or means for eX- tracting electron beams which I have shown and described here operates on a principle different from that shown in my above-mentioned Patent 2,394,070. In the structure shown in that patent it was necessary to time and relate a magnetizing current on the electromagnetic defiecting arrangement in a properly coordinated time and amplitude relationship to the rapidly changing main eld through the tube, and this presents many diiiiculties in practice. By the arrangement shown here I provide an arrangement which does not require any special energization, timing or otherwise, and which operates by getting rid of the magnetic field which would otherwise be present in a certain space in the tube, rather` than by trying to create a diierent magnetic field, the present arrangement being very simple and effective in operation.

While I have shown and described certain embodiments of my invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as found in the appended claims.

I claim:

1. A magnetic induction accelerator comprising a closed vessel having therein a deecting member of material of high magnetic permeability, said member having a portion with a slot in the plane of theelectron orbit and a portion in the same plane with a thickness of the same order as the depth of said slot, the slot in said member providing a space of greatly reduced field strength relative to adjacent areas. l

2. Apparatus of the character claimed in claim 1, wherein said member is formed of laminations lying in planes perpendicular to that of said orbit and generally radial thereof.

3. A magnetic induction accelerator comprising a closed vessel having therein a deecting member of material of high magnetic permeability having at least a portion in the plane of the electron orbit, said member being formed of laminations lying in planes perpendicular to that of said orbit and generally radial thereof, said portion being thick enough and having other portions extending at least sumciently far therefrom to always disturb the symmetry of the accelerating field and to provide a space in the plane of the orbit immediately adjacent said first mentioned deiiecting portion wherein the accelerating field has a strength less than half that in other adjacent areas in said plane.

4. A magnetic induction accelerator comprising a closed vessel having therein a deflecting member of material of high magnetic permeability, said member having a portion with a slot in the plane of the electron orbit and an immediately adjacent portion of substantial thickness, the Walls of said slot being substantially straight to provide a space of greatly reduced field strength relative to adjacent areas.

DONALD W. KERST.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS 

